Solid scintillator, radiation detector, and x-ray tomographic imaging apparatus

ABSTRACT

The solid scintillator according to the present invention is expressed by the following formula (1): 
       [Formula 1] 
       (M 1-x-y Gd x Ce y ) 3 J 5 O 12   (1)
 
     (wherein M is at least one element of La and Tb; J is at least one metal selected from the group consisting of Al, Ga, and In; and x and y are such that 0.5≦x≦1 and 0.000001≦y≦0.2). The transmittance of light having a wavelength of 550 nm measured at a thickness of 2 mm is equal to or greater than 40%. The solid scintillator according to the present invention can be manufactured at low cost, has a high light emitting power, and does not release Cd because Cd is not contained.

TECHNICAL FIELD

The present invention relates to a technique for converting radiationsuch as X-rays to visible light, and more particularly to a solidscintillator, a radiation detector using this solid scintillator, and anx-ray tomographic imaging apparatus.

BACKGROUND ART

Inspection using a radiographic inspection apparatus such as an x-raytomographic imaging apparatus (X-ray CT scanner) is performed in medicaldiagnosis, industrial inspection, and security fields. In general, theX-ray CT scanner includes an X-ray tube (X-ray source) which emits afan-beam X-ray or a fan-shaped x-ray beam; an X-ray detector arrangedfacing the X-ray tube and having a large number of X-ray detectionelements; and an image reconstruction apparatus which reconstructs animage based on data from the X-ray detector. An object is placed betweenthe X-ray tube and the X-ray detector, and a tomographic plane thereofis imaged by fan-beam X-ray radiation.

The X-ray CT scanner emits a fan-beam X-ray and collects X-rayabsorption data. The X-ray CT scanner repeats this processing bychanging the radiation angle to the tomographic plane, for example, eachby one degree. Then, the X-ray CT scanner analyzes the obtained data bya computer to calculate the X-ray absorption rate on the tomographicplane of the object and constructs an image of the tomographic planeaccording to the X-ray absorption rate.

As the X-ray detector of the X-ray CT scanner, there is used a solidscintillator which emits visible light by X-ray stimulation. The solidscintillator refers to a scintillator made of ceramic or single crystal.

It is preferable to use a solid scintillator as the X-ray detector ofthe X-ray CT scanner because the use of the solid scintillator as theX-ray detector can reduce the size of a detection element and thus caneasily increase the number of channels for higher resolution.

As the solid scintillator used as a radiation detector such as the X-raydetector, conventionally there are known a single crystal such ascadmium tungstate (CdWO₄), sodium iodide (NaI), and cesium iodide (CsI);europium-activated barium fluorochloride (BaFCl:Eu); terbium-activatedlanthanum oxybromide (LaOBr:Tb); thallium-activated cesium iodide(CsI:Tl); calcium tungstate (CaWO₄); cadmium tungstate (CdWO₄);praseodymium-activated gadolinium oxysulfide (Gd₂O₂S:Pr) disclosed inJapanese Patent Laid-Open No. S58-204088 (Patent document 1); and thelike.

Patent Document 1: Japanese Patent Laid-Open No. S58-204088

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

Of them, the praseodymium-activated gadolinium oxysulfide (Gd₂O₂S:Pr)disclosed in Patent document 1 is preferable as the X-ray detectionscintillator because the praseodymium-activated gadolinium oxysulfide(Gd₂O₂S:Pr) has a large X-ray absorption coefficient and a shortfluorescence decay time. In particular, when the X-ray CT scanner isused in a medical field, the praseodymium-activated gadoliniumoxysulfide (Gd₂O₂S:Pr) having a large X-ray absorption coefficient and ashort fluorescence decay time is preferable in terms of reduction inexposure amount and diagnosis time.

However, rare-earth oxysulfide ceramics such as praseodymium-activatedgadolinium oxysulfide needs to be manufactured using HIP (hot isostaticpressing) and thus has a problem with higher manufacturing costs thanusing hot pressing (uni-axially pressing) or vacuum sintering. Moreover,when the X-ray CT scanner is used in a security field such asbaggage-screening devices, the detection element has a large area, andthus the scintillator is not required very much to have a large X-rayabsorption coefficient or a short fluorescence decay time. Therefore,the praseodymium-activated gadolinium oxysulfide has a big problem withhigh manufacturing costs.

In contrast to this, cadmium tungstate (CdWO4) single crystalscintillator is inferior to the rare-earth oxysulfide ceramicscintillator such as praseodymium-activated gadolinium oxysulfide interms of characteristics but superior in terms of costs, and thus ispreferable as the scintillator for an X-ray CT scanner in the securityfield. However, Cd is a hazardous substance and thus has a problem thatthe cadmium tungstate (CdWO₄) single crystal scintillator has a risk ofdeteriorating the environment.

In view of the above circumstances, the present invention has been made,and an object of the present invention is to provide a solidscintillator which can be manufactured at low cost, has high lightemitting power, and does not contain Cd, a radiation detector using thissolid scintillator, and an x-ray tomographic imaging apparatus.

Means for Solving the Problems

The solid scintillator according to an aspect of the present inventionis to solve the above problems and is expressed by the following formula(1).

[Formula 1]

(M_(1-x-y)Gd_(x)Ce_(y))₃J₅O₁₂  (1)

(wherein M is at least one element of La and Tb; J is at least one metalselected from the group consisting of Al, Ga, and In; and x and y aresuch that 0.5≦x≦1 and 0.000001≦y≦0.2).

The transmittance of light having a wavelength of 550 nm measured at athickness of 2 mm is equal to or greater than 40%.

Moreover, the solid scintillator according to another aspect of thepresent invention is to solve the above problems and is expressed by thefollowing formula (2).

[Formula 2]

(M_(1-x-y)Gd_(x)Pr_(y))₃J₅O₁₂  (2)

(wherein M is at least one element of La and Tb; J is at least one metalselected from the group consisting of Al, Ga, and In; and x and y aresuch that 0.5≦x≦1 and 0.000001≦y≦0.2).

The transmittance of light having a wavelength of 610 nm measured at athickness of 2 mm is equal to or greater than 40%.

Further, the radiation detector according to another aspect of thepresent invention is to solve the above problems and uses the solidscintillator.

Moreover, the x-ray tomographic imaging apparatus according to anotheraspect of the present invention is to solve the above problems and usesthe radiation detector.

ADVANTAGEOUS EFFECTS OF THE INVENTION

The solid scintillator according to the present invention can provide asolid scintillator which can be manufactured at low cost, has a highlight emitting power, and does not contain Cd.

Moreover, the radiation detector according to the present invention canprovide a radiation detector which can be manufactured at low cost, hasa high light emitting power, and there is no risk of releasing Cd at itsdisposal.

Further, the x-ray tomographic imaging apparatus according to thepresent invention can provide an x-ray tomographic imaging apparatuswhich can be manufactured at low cost, has a small amount of exposure,and there is no risk of releasing Cd at its disposal.

BEST MODE FOR CARRYING OUT THE INVENTION

The solid scintillator according to the present invention includes asolid scintillator containing Ce as the activator (hereinafter referredto as a “first solid scintillator”) and a solid scintillator containingPr as the activator (hereinafter referred to as a “second solidscintillator”).

[First Solid Scintillator]

The first solid scintillator is a rare-earth aluminate compound having acomposition expressed by the following formula (1).

[Formula 3]

(M_(1-x-y)Gd_(x)Ce_(y))₃J₅O₁₂  (1)

In the formula (1), M is at least one element of La and Tb.

In the formula (1), J is at least one metal selected from the groupconsisting of Al, Ga, and In.

In the formula (1), x is such that 0.5≦x≦1. When x is such that 0.5≦x≦1,that is, Gd content of the total amount of M, Gd, and Ce is equal to orgreater than 50 mol % and equal to or less than 100 mol %, thescintillator has a high X-ray absorption coefficient and light-emittingefficiency, and thus is preferable.

When x is less than 0.5, that is, Gd content of the total amount of M,Gd, and Ce is less than 50 mol %, X-ray cannot be sufficiently absorbedin the scintillator and thus there is a risk of lowering the lightemitting power.

In the formula (1), y is 0.000001≦y≦0.2, and preferably 0.001≦y≦0.1.

Ce is an activator for increasing the light-emitting efficiency of therare-earth aluminate compound expressed by the formula (1). When y is0.000001≦y≦0.2, that is, Ce content of the total amount of M, Gd, and Ceis equal to or greater than 0.0001 mol% and equal to or less than 20 mol%, the scintillator has a high light-emitting efficiency and thus ispreferable.

When y is less than 0.000001 or exceeds 0.2, that is, Ce content of thetotal amount of M, Gd, and Ce is less than 0.0001 mol %, the content ofCe contributing to light-emitting runs short, and thus thelight-emitting efficiency becomes low. Meanwhile, when y exceeds 20 mol%, the material constituting the scintillator is colored and thustransparency becomes low. Therefore, there is a risk that a sufficientlight emitting power cannot be obtained.

The first solid scintillator may be either a single crystal orpolycrystalline ceramics.

The first solid scintillator has a high X-ray absorption coefficient andlight-emitting efficiency. Therefore, when a scintillator with athickness of 2 mm is used, the transmittance of light, which is emittedfrom Ce by X-ray irradiation and has a wavelength of 550 nm, becomesequal to or greater than 40%.

Here, the transmittance means a linear transmittance. The lineartransmittance is an indication of transparency of the first solidscintillator made of a rare-earth aluminate compound. In the case wherethe linear transmittance (transparency) of the first solid scintillatorbecomes higher, the more light reaches the photoreceiver such as aphotodiode. Therefore, when the linear transmittance becomes higher, thelight output increases more.

When a scintillator having a transmittance of 40% or higher is used asthe solid scintillator, the light output can be considered to besufficiently high.

The first solid scintillator can be manufactured, for example, in such amanner that a rare-earth oxide powder is manufactured by firing a mixedpowder prepared by mixing oxides such as Tb₄O₇, La₂O₃, Gd₂O₃, Al₂O₃,Ga₂O₃, CeO₂ and the like under an inert gas atmosphere such as Ar andthe like; the rare-earth oxide powder is molded by rubber pressing orthe like; and then the molded body is heated by hot pressing(uni-axially pressing) or vacuum sintering until densification isattained.

The conventional rare-earth oxysulfide ceramics such as thepraseodymium-activated gadolinium oxysulfide is pyrolyzed at hightemperature, and thus needs to be manufactured by HIP (hot isostaticpressing) using a metal capsule such as Ta. On the contrary, the firstsolid scintillator according to the present invention is not pyrolyzedat high temperature, and thus does not need to use a metal capsule.Therefore, the first solid scintillator can be manufactured by hotpressing or vacuum sintering simpler and lower in manufacturing costthan the HIP.

The heat processing using hot pressing and vacuum sintering is usuallyperformed at temperatures 1400° C. to 1700° C. and for one to 10 hours.

When the heating and pressing using hot pressing is performed, at leastthe atmosphere in a processing chamber in the hot pressing processor ispreferably replaced with an inert gas such as an argon gas.

The first solid scintillator can be manufactured by hot pressing orvacuum sintering at low cost, has a high light emitting power, and canprovide a solid scintillator which does not contain Cd.

[Second Solid Scintillator]

The second solid scintillator is a rare-earth aluminate compound havinga composition expressed by the following formula (2).

[Formula 4]

(M_(1-x-y)Gd_(x)Pr_(y))₃J₅O₁₂  (2)

In the formula (2), M and J are the same as in the above formula (1) andthus the description is omitted.

In the formula (2), x is such that 0.5≦x≦1. When x is 0.5≦x≦1, that is,Gd content of the total amount of M, Gd, and Pr is equal to or greaterthan 50 mol % and equal to or less than 100 mol %, the scintillator hasa high X-ray absorption coefficient and light-emitting efficiency andthus is preferable.

When x is less than 0.5, Gd content of the total amount of M, Gd, and Pris less than 50 mol %, X-ray cannot be sufficiently absorbed in thescintillator and thus there is a risk that the light emitting powerbecomes low.

In the formula (2), y is 0.000001≦y≦0.2, and preferably 0.0001≦y≦0.01.

Pr is an activator for increasing the light-emitting efficiency of therare-earth aluminate compound expressed by the formula (2). When y is0.000001≦y≦0.2, that is, Pr content of the total amount of M, Gd, and Pris equal to or greater than 0.0001 mol % and equal to or less than 20mol %, the scintillator has a high light-emitting efficiency and thus ispreferable.

When y is less than 0.000001 or exceeds 0.2, that is, Pr content of thetotal amount of M, Gd, and Pr is less than 0.0001 mol %, the content ofPr contributing to light-emitting runs short, and thus thelight-emitting efficiency becomes low. Meanwhile, when y exceeds 20 mol%, the material constituting the scintillator is colored and thustransparency becomes low. Therefore, there is a risk that a sufficientlight emitting power cannot be obtained.

The second solid scintillator may be either a single crystal orpolycrystalline ceramics.

The second solid scintillator has a high X-ray absorption coefficientand light-emitting efficiency. Therefore, when a scintillator with athickness of 2 mm is used, the transmittance of light, which is emittedfrom Pr by X-ray irradiation and has a wavelength of 610 nm, becomesequal to or greater than 40%. When a scintillator having a transmittanceof 40% or higher is used as the solid scintillator, the light output canbe considered to be sufficiently high.

The second solid scintillator can be manufactured, for example, in sucha manner that a rare-earth oxide powder is manufactured by firing amixed powder prepared by mixing oxides such as Tb₄O₇, La₂O₃, Gd₂O₃,Al₂O₃, Ga₂O₃, Pr₇O₁₁ and the like under an inert gas atmosphere such asAr and the like; and the rare-earth oxide powder is processed by hotpressing (uni-axially pressing) or vacuum sintering in the same manneras for the first solid scintillator until densification is attained.

The heat processing using hot pressing and vacuum sintering is usuallyperformed at temperatures 1400° C. to 1700° C. and for one to 10 hours.

When the heating and pressing using hot pressing is performed, at leastthe atmosphere in a processing chamber in the hot pressing processor ispreferably replaced with an inert gas such as an argon gas.

The second solid scintillator can be manufactured by hot pressing orvacuum sintering at low cost, has a high light emitting power, and canprovide a solid scintillator which does not contain Cd.

The radiation detector according to the present invention is such thatthe solid scintillator according to the present invention is used, forexample, as the X-ray detection element of the X-ray detector.

The radiation detector according to the present invention can beconfigured, for example, to include the solid scintillator and aphotomultiplier tube which converts light emitted from the solidscintillator to electrical energy.

The radiation detector according to the present invention uses the solidscintillator according to the present invention, and thus can provide aradiation detector which can be manufactured at lower cost, has a higherlight output and there is no risk of releasing Cd at its disposal incomparison with the use of the conventional solid scintillator.

The x-ray tomographic imaging apparatus according to the presentinvention is such that the solid scintillator according to the presentinvention is used as the X-ray detection element of the X-ray detector.

The x-ray tomographic imaging apparatus according to the presentinvention can be configured, for example, to include an X-ray tube, anX-ray detector using the solid scintillator according to the presentinvention, and an image reconstruction apparatus which reconstructs animage based on data from the X-ray detector.

The x-ray tomographic imaging apparatus according to the presentinvention uses the solid scintillator according to the presentinvention, and thus can provide an x-ray tomographic imaging apparatuswhich can be manufactured at lower cost, can reduce the amount of X-rayexposure and there is no risk of releasing Cd at its disposal incomparison with the use of the conventional solid scintillator.

EXAMPLES

Hereinafter, examples are given, but it should not be construed that thepresent invention is limited to the examples.

Example 1

Gd₂O₃, Al₂O₃ and CeO₂ were powder-mixed at a predetermined compositionratio to obtain a total weight of 500 g and then fired at 1200° C. underan Ar gas atmosphere. Then, rare-earth oxides phosphor powder with anaverage particle diameter D₅₀ of 5.0 μm expressed by(Gd_(0.99)Ce_(0.01))₃Al₅O₁₂(Ce concentration of 1 mol %) was obtained.

The phosphor powder was molded by rubber pressing and the obtainedmolded body was set within a carbon mold of the hot pressing(uni-axially pressing) processor. Argon gas was filled as a pressingmedium into the hot pressing processor and was subjected to a pressure(surface pressure) of 49 MPa and a temperature of 1700° C. for threehours. Then, a sintered body expressed by (Gd_(0.99)Ce_(0.01))₃Al₅O₁₂(Ceconcentration of 1 mol %) was obtained. The sintered body wasmechanically processed with a multi-wire saw to fabricate a ceramicscintillator with 25 mm high×25 mm wide×2 mm thick.

When an X-ray with a tube voltage of 120 Kvp is emitted, the ceramicscintillator emits visible light with a peak wavelength near 550 nm. Forthis reason, the linear transmittance of light with a wavelength of 550nm was measured as the indication of transparency of the ceramicscintillator. Specifically, a surface of 25 mm×25 mm square of theceramic scintillator was irradiated with light containing a wavelengthof 550 nm. Then, the linear transmittance of light with a wavelength of550 nm (hereinafter the linear transmittance is referred to simply as“transmittance”) was measured at a thickness of 2 mm. The transmittancewas 52%.

Moreover, the light output of the ceramic scintillator was measured.Specifically, in order to block soft X-rays, a 120 Kvp X-ray wastransmitted through a 20 mm Al filter and was emitted to one surface of25 mm×25 mm square of the ceramic scintillator. Then, the value ofcurrent flowing through a silicon photo diode provided on a surface onthe rear side of the one surface was calculated as the light output. Thelight output of CdWO₄ as a comparative sample was also measured underthe same conditions. The light output of the ceramic scintillator wascalculated as a relative light output value (%) assuming that the lightoutput of CdWO₄ was 100%. The relative light output value (%) was 128%.

The results are shown in Table 1.

Example 2

Tb₄O₇, Gd₂O₃, Al₂O₃ and CeO₂ were powder-mixed at a predeterminedcomposition ratio to obtain a total weight of 500 g and then fired at1200° C. under an Ar gas atmosphere. Then, rare-earth oxides phosphorpowder with an average particle diameter D₅₀ of 4.8 μm expressed by(Tb_(0.49)Gd_(0.5)Ce_(0.01))₃Al₅O₁₂ (Ce concentration of 1 mol %) wasobtained.

A sintered body expressed by (Tb_(0.49) Gd_(0.5)Ce_(0.01))₃Al₅O₁₂ (Ceconcentration of 1 mol %) in the same manner as in the Example 1 exceptthe use of the phosphor powder was obtained and a ceramic scintillatorwas fabricated.

The transmittance and the light output of the obtained ceramicscintillator were measured in the same manner as in the Example 1. Thetransmittance was 63% and the relative light output value (%) was 142%.The results are shown in Table 1.

Example 3

La₂O₃, Gd₂O₃, Al₂O₃ and CeO₂ were powder-mixed at a predeterminedcomposition ratio to obtain a total weight of 500 g and then fired at1200° C. under an Ar gas atmosphere. Then, rare-earth oxides phosphorpowder with an average particle diameter D₅₀ of 5.2 μm expressed by(La_(0.49)Gd_(0.5)Ce_(0.01))₃Al₅O₁₂ (Ce concentration of 1 mol %) wasobtained.

A sintered body expressed by (La_(0.49)Gd_(0.5)Ce_(0.01))₃Al₅O₁₂ (Ceconcentration of 1 mol %) in the same manner as in the Example 1 exceptthe use of the phosphor powder was obtained and a ceramic scintillatorwas fabricated.

The transmittance and the light output of the obtained ceramicscintillator were measured in the same manner as in the Example 1. Thetransmittance was 48% and the relative light output value (%) was 117%.The results are shown in Table 1.

Example 4

Tb₄O₇, Gd₂O₃, Ga₂O₃ and CeO₂ were powder-mixed at a predeterminedcomposition ratio to obtain a total weight of 500 g and then fired at1200° C. under an Ar gas atmosphere. Then, rare-earth oxides phosphorpowder with an average particle diameter D₅₀ of 4.8 μm expressed by(Tb_(0.3)Gd_(0.6)Ce_(0.1))₃Ga₅O₁₂ (Ce concentration of 10 mol %) wasobtained.

A sintered body expressed by (Tb_(0.3)Gd_(0.6)Ce_(0.1))₃Ga₅O₁₂ (Ceconcentration of 10 mol %) in the same manner as in the Example 1 exceptthe use of the phosphor powder was obtained and a ceramic scintillatorwas fabricated.

The transmittance and the light output of the obtained ceramicscintillator were measured in the same manner as in the Example 1. Thetransmittance was 58% and the relative light output value (%) was 138%.The results are shown in Table 1.

Example 5

Gd₂O₃, Al₂O₃ and Pr₇O₁₁ were powder-mixed at a predetermined compositionratio to obtain a total weight of 500 g and then fired at 1200° C. underan Ar gas atmosphere. Then, rare-earth oxides phosphor powder with anaverage particle diameter D₅₀ of 5.0 μm expressed by(Gd_(0.99)Pr_(0.01))₃Al₅O₁₂ (Pr concentration of 1 mol %) was obtained.

A sintered body expressed by (Gd_(0.99)Pr_(0.01))₃Al₅O₁₂ (Prconcentration of 1 mol %) in the same manner as in the Example 1 exceptthe use of the phosphor powder was obtained and a ceramic scintillatorwas fabricated.

When an X-ray with a tube voltage of 120 Kvp is emitted, the ceramicscintillator emits visible light with a peak wavelength near 610 nm. Forthis reason, the transmittance of the obtained ceramic scintillator wasmeasured in the same manner as in the Example 1 except the use of lightwith a wavelength of 610 nm instead of light with a wavelength of 550nm. Moreover, the light output of the ceramic scintillator was measuredin the same manner as in the Example 1. The transmittance was 47% andthe relative light output value (%) was 115%. The results are shown inTable 1.

Example 6

Tb₄O₇, Gd₂O₃, Ga₂O₃ and Pr₇O₁₁ were powder-mixed at a predeterminedcomposition ratio to obtain a total weight of 500 g and then fired at1200° C. under an Ar gas atmosphere. Then, rare-earth oxides phosphorpowder with an average particle diameter D₅₀ of 4.8 μm expressed by(Tb_(0.03)Gd_(0.69)Pr_(0.01))₃Ga₅O₁₂ (Pr concentration of 1 mol %) wasobtained.

A sintered body expressed by (Tb_(0.3)Gd_(0.69)Pr_(0.01))₃Ga₅O₁₂ (Prconcentration of 1 mol %) in the same manner as in the Example 1 exceptthe use of the phosphor powder was obtained and a ceramic scintillatorwas fabricated.

When an X-ray with a tube voltage of 120 Kvp is emitted, the ceramicscintillator emits visible light with a peak wavelength near 610 nm. Forthis reason, the transmittance of the obtained ceramic scintillator wasmeasured in the same manner as in the Example 1 except the use of lightwith a wavelength of 610 nm instead of light with a wavelength of 550nm. Moreover, the light output of the ceramic scintillator was measuredin the same manner as in the Example 1. The transmittance was 57% andthe relative light output value (%) was 132%. The results are shown inTable 1.

Comparative Example 1

Gd₂O₃, Al₂O₃ and CeO₂ were powder-mixed at a predetermined compositionratio to obtain a total weight of 500 g and then fired at 1200° C. underan Ar gas atmosphere. Then, rare-earth oxides phosphor powder with anaverage particle diameter D₅₀ of 5.0 μm expressed by(Gd_(0.7)Ce_(0.3))₃Al₅O₁₂ (Ce concentration of 30 mol %) was obtained.

A sintered body expressed by (Gd_(0.7)Ce_(0.3))₃Al₅O₁₂ (Ce concentrationof 30 mol %) in the same manner as in the Example 1 except the use ofthe phosphor powder was obtained and a ceramic scintillator wasfabricated.

The transmittance and the light output of the obtained ceramicscintillator were measured in the same manner as in the Example 1. Thetransmittance was 12% and the relative light output value (%) was 18%.The results are shown in Table 1.

Comparative Example 2

La₂O₃, Gd₂O₃, Al₂O₃ and CeO₂ were powder-mixed at a predeterminedcomposition ratio to obtain a total weight of 500 g and then fired at1200° C. under an Ar gas atmosphere. Then, rare-earth oxides phosphorpowder with an average particle diameter D₅₀ of 5.2 μm expressed by(La_(0.59)Gd_(0.4)Ce_(0.01))₃Al₅O₁₂ (Ce concentration of 1 mol %) wasobtained.

A sintered body expressed by (La_(0.59)Gd_(0.4)Ce_(0.01))₃Al₅O₁₂ (Ceconcentration of 1 mol %) in the same manner as in the Example 1 exceptthe use of the phosphor powder was obtained and a ceramic scintillatorwas fabricated.

The transmittance and the light output of the obtained ceramicscintillator were measured in the same manner as in the Example 1. Thetransmittance was 38% and the relative light output value (%) was 87%.The results are shown in Table 1.

Comparative Example 3

Tb₄O₇, Gd₂O₃, Al₂O₃ and CeO₂ were powder-mixed at a predeterminedcomposition ratio to obtain a total weight of 500 g and then fired at1200° C. under an Ar gas atmosphere. Then, rare-earth oxides phosphorpowder with an average particle diameter D₅₀ of 4.8 μm expressed by(Tb_(0.79)Gd_(0.2)Ce_(0.01))₃Al₅O₁₂ (Ce concentration of 1 mol %) wasobtained.

A sintered body expressed by (Tb_(0.79)Gd_(0.2)Ce_(0.01))₃Al₅O₁₂ (Ceconcentration of 1 mol %) in the same manner as in the Example 1 exceptthe use of the phosphor powder was obtained and a ceramic scintillatorwas fabricated.

The transmittance and the light output of the obtained ceramicscintillator were measured in the same manner as in the Example 1. Thetransmittance was 30% and the relative light output value (%) was 80%.The results are shown in Table 1.

Comparative Example 4

Gd₂O₃, Ga₂O₃ and CeO₂ were powder-mixed at a predetermined compositionratio to obtain a total weight of 500 g and then fired at 1200° C. underan Ar gas atmosphere. Then, rare-earth oxides phosphor powder with anaverage particle diameter D₅₀ of 5.1 μm expressed by(Gd_(0.7)Ce_(0.3))₃Ga₅O₁₂ (Ce concentration of 30 mol %) was obtained.

A sintered body expressed by (Gd_(0.7)Ce_(0.3))₃Ga₅O₁₂ (Ce concentrationof 30 mol %) in the same manner as in the Example 1 except the use ofthe phosphor powder was obtained and a ceramic scintillator wasfabricated.

The transmittance and the light output of the obtained ceramicscintillator were measured in the same manner as in the Example 1. Thetransmittance was 21% and the relative light output value (%) was 54%.The results are shown in Table 1.

Comparative Example 5

Gd₂O₃, Al₂O₃ and Pr₇O₁₁ were powder-mixed at a predetermined compositionratio to obtain a total weight of 500 g and then fired at 1200° C. underan Ar gas atmosphere. Then, rare-earth oxides phosphor powder with anaverage particle diameter D₅₀ of 5.0 μm expressed by(Gd_(0.7)Pr_(0.3))₃Al₅O₁₂ (Pr concentration of 30 mol %) was obtained.

A sintered body expressed by (Gd_(0.7)Pr_(0.3))₃Al₅O₁₂ (Pr concentrationof 30 mol %) in the same manner as in the Example 1 except the use ofthe phosphor powder was obtained and a ceramic scintillator wasfabricated.

When an X-ray with a tube voltage of 120 Kvp is emitted, the ceramicscintillator emits visible light with a peak wavelength near 610 nm. Forthis reason, the transmittance of the obtained ceramic scintillator wasmeasured in the same manner as in the Example 1 except the use of lightwith a wavelength of 610 nm instead of light with a wavelength of 550nm. Moreover, the light output of the ceramic scintillator was measuredin the same manner as in the Example 1. The transmittance was 16% andthe relative light output value (%) was 27%. The results are shown inTable 1.

TABLE 1 Transmittance of Transmittance of Light having Light havingComposition of the Wavelength of Wavelength of Relative Light SolidScintillator 550 nm % 610 nm % Output Value Example 1(Gd_(0.99)Ce_(0.01))₃Al₅O₁₂ 52 — 128 Example 2(Tb_(0.49)Gd_(0.5)Ce_(0.01))₃Al₅O₁₂ 63 — 142 Example 3(La_(0.49)Gd_(0.5)Ce_(0.01))₃Al₅O₁₂ 48 — 117 Example 4(Tb_(0.3)Gd_(0.6)Ce_(0.1))₃Ga₅O₁₂ 58 — 138 Example 5(Gd_(0.99)Pr_(0.01))₃Al₅O₁₂ — 47 115 Example 6(Tb_(0.3)Gd_(0.69)Pr_(0.01))₃Ga₅O₁₂ — 57 132 Comparative(Gd_(0.7)Ce_(0.3))₃Al₅O₁₂ 12 —  18 Example1 Comparative(La_(0.59)Gd_(0.4)Ce_(0.01))₃Al₅O₁₂ 38 —  87 Example 2 Comparative(Tb_(0.79)Gd_(0.2)Ce_(0.01))₃Al₅O₁₂ 30 —  80 Example 3 Comparative(Gd_(0.7)Ce_(0.3))₃Ga₅O₁₂ 21 —  54 Example 4 Comparative(Gd_(0.7)Pr_(0.3))₃Al₅O₁₂ — 16  27 Example 5

INDUSTRIAL APPLICABILITY

The solid scintillator according to the present invention can be usedin, for example, a solid scintillator constituting a radiographicinspection apparatus for use in medical diagnosis, industrialinspection, and security fields and, specifically, a solid scintillatorconstituting a radiation detector, an x-ray tomographic imagingapparatus, and the like.

The radiation detector according to the present invention can be usedin, for example, a radiation detector for use in medical diagnosis,industrial inspection, and security fields.

The x-ray tomographic imaging apparatus according to the presentinvention can be used in, for example, an x-ray tomographic imagingapparatus for use in medical diagnosis, industrial inspection, andsecurity fields.

1. A solid scintillator represented by the following formula (1):(M_(1-x-y)Gd_(x)Ce_(y))₃J₅O₁₂  (1) wherein M is at least one element ofLa and Tb; J is at least one metal selected from the group consisting ofAl, Ga, and In; and x and y are such that 0.5≦x≦1 and 0.000001≦y≦0.2,wherein transmittance of light having a wavelength of 550 nm measured ata thickness of 2 mm is equal to or greater than 40%.
 2. A solidscintillator expressed by the following formula (2):(M_(1-x-y)Gd_(x)Pr_(y))₃J₅O₁₂  (2) wherein M is at least one element ofLa and Tb; J is at least one metal selected from the group consisting ofAl, Ga, and In; and x and y are such that 0.5≦x≦1 and 0.000001≦y≦0.2,wherein transmittance of light having a wavelength of 610 nm measured ata thickness of 2 mm is equal to or greater than 40%.
 3. A radiationdetector comprising the solid scintillator according to claim
 1. 4. Anx-ray tomographic imaging apparatus comprising the radiation detectoraccording to claim 3.